With exceptional materials properties, carbon nanotubes offer tremendous opportunities for scientific and technological exploration. Since their discovery more than a decade ago their use in electronics applications continues to grow, as exemplified by CNT-based field-effect-transistors (FETs) memory devices and tunable oscillators.
Recently, carbon nanotubes have also been applied to pressure sensing based on piezoresistance, opening up yet another promising application of nanotubes. For piezoresistance pressure sensing devices, a metallic single-walled-nanotube is placed on a membrane of atomic-layer-deposited (ALD) alumina. A pressure differential across the membrane causes the membrane to bulge, inducing strain in the overlying single-walled-nanotube. Strain-induced conductivity changes of <12% in non-vacuum environments (760-900 Torr) have been reported.
Other pressure measuring devices are available. Popular thermal conductivity gauges not employing single-walled-nanotubes such as Pirani or thermocouple gauges operate on principles of thermal exchange between a voltage-biased element and the surrounding gas. Such gauges are physically large (volume ˜10 cm3), prohibiting their integration with many vacuum-encased Si-based micro-cavity applications (vacuum microelectronics, micro-eletromechanical-systems (MEMS) such as gyroscopes and RF switches). As a result of their large thermal mass, such thermocouple gauges are inherently slow, and also operate at substantial power levels and high temperatures.
With the advent of MEMS technology, miniaturized, low power pressure sensors have been developed, a first application of bulk Si-micromachining. Many of these Si-based transducers operate on the piezoresistive or capacitive principle. Obtaining wide dynamic range with these transducers has been challenging; and yet achieving wide dynamic range is especially important in micro-cavity applications. Due to the small volumes in micro-cavities, outgassing can cause large pressure changes over short periods, affecting device performance and requiring monitoring by sensors capable of wide dynamic range.